The Patriots have gained the most yards in the league and are in the top seven through the air and on the ground. The 49ers have allowed the second-fewest yards overall with a balanced defense: second against the pass and tied for second against the run.

So when two of the top Super Bowl contenders meet, with both hoping to move closer to a first-round bye and home-field advantage throughout their conference playoffs, it will be strength against strength.

The Patriots (10-3) have clinched the AFC East and are tied with Denver for the second-best record in the conference, but hold the tiebreaker because they beat the Broncos. The 49ers (9-3-1) have the second-best mark in the NFC and can lock up first place in the West by beating the Patriots if Seattle loses to Buffalo. Even if the Seahawks win, the 49ers clinch a playoff spot if they beat the Patriots.

That won't be easy.

Brady has made lots of superior defenses look soft.

Just last Monday night, he threw touchdown passes on his first three series and another in the third quarter in a 42-14 rout of Houston. Having the NFL's best record at the time and the AFC's leading sacker, J.J. Watt, didn't help the Texans once Brady went to work.

The 49ers have Smith, whose 19 1-2 sacks are just three shy of the single-season record set by Michael Strahan with the New York Giants in 2001.

"There's no real weakness on this defense and they do everything well," Brady said. "You have to do everything well, too."

He usually does, especially in December. The Patriots have won their last 20 games at home in that month.

The 49ers, meanwhile, have a quarterback making just his fifth NFL start. The mobile Colin Kaepernick had strong performances in his first two starts, but the offense struggled in the past two games despite having the league's second-ranked rushing attack. His inexperience could hurt against an improving defense.

"He hasn't played in many games, but he's been very productive," Patriots safety Devin McCourty said.

It's on defense where the 49ers excel.

They've allowed just 14.2 points per game and haven't given up more than 26. The Patriots are averaging 19.5 in the first half alone and have scored more than 26 in 10 games.

And New England has Wes Welker, who needs five catches to become the first receiver with at least 100 in five seasons.

"We all have to go out there and win out one-on-one battles," cornerback Tarell Brown said. "They have a lot of different weapons, a lot of guys that can make plays, but we're really fighting against Tom Brady. He has the keys to the car and we definitely have to stop him."

The 49ers must guard against being fooled. Brady is outstanding on play-action passes like the 37-yard touchdown he threw to a wide-open Brandon Lloyd against the Texans. With Stevan Ridley in the midst of a 1,000-yard rushing season, New England's running game is dangerous, too.

"Tom Brady's mechanics are really good and they do a good job disguising things," Smith said, "so you have to not only be disciplined but try to anticipate and pay attention to the film so, come game time, we're not caught off-guard by anything."

The two-time MVP quarterback is having another brilliant season with 29 touchdown passes and four interceptions. He's been remarkably consistent, completing at least 60 percent of his passes in every game, and been sacked only 20 times.

But here comes Smith.

"I face a bunch of good pass rushers," Patriots left tackle Nate Solder said. "It seems like every week it's a top pass rusher. So I guess that's the excitement of playing in the NFL."

Smith was drafted with the seventh pick in 2011, just four spots before Watt, and both have made powerful impacts. But Monday night, Watt was held without a sack for only the third time this season after getting six in his previous three games.

What worked against the Texans, though, may not work against the 49ers.

"They're different schemes and different players," Brady said. "I think they both rush the passer well. I think they're both good in coverage. That's why you win a lot of games, because you're really fundamentally sound in all of those areas. But in terms of style, they are very different teams. But it's equally as big of a challenge for us."

For the 49ers, too. They can't focus too much on the passing game, not with Ridley leading the NFL's seventh best running game with 1,082 yards.

So too much pressure on the quarterback can be a bad thing.

"Any team that just goes out there and blitzes away is going to get blown out," San Francisco safety Donte Whitner said. "You can't do that. You have to mix it up. Any quarterback that's this good — that's an A-plus at the helm — you have to mix some things. Everybody knows that you can't go out there and do one thing. You have to do multiple things. With our personnel and our scheme, I believe that allows us to do it."